GIFT   OF 


THE  LAW  OF  IRREVERSIBLE  EVOLUTION 


BY 


BRANISLAV  PETRONIEVICS,  PH.  D. 


FROM  THE  SMITHSONIAN  REPORT  FOR  1918,  PAGES  42&-440 


(PUBLICATION  2565) 


WASHINGTON 

GOVERNMENT  TRINTING   OFFICE 
1920 


ON  THE  LAW  OF  IRREVERSIBLE  EVOLUTION 


BY 


BRANISLAV  PETRONIEVICS,  PH.  1). 


FROM  THE  SMITHSONIAN  REPORT  FOR  1918,  PAGES  429-440 


(  PUBLICATION  2565) 


WASHINGTON 

GOVERNMENT   PRINTING    OFFICE 
1920 


ON  THE  LAW  OF  IRREVERSIBLE  EVOLUTION. 


BY    BllANISLAV    ]'KTKO:\IEV-LCS,    I'll.    D. 


Louis  Dollo,  the  great  Belgian  paleontologist,  first  publicly  for- 
mulated in  1893  (Dollo,  4)  his  famous  law  of  irreversible  evolution, 
one  of  the  most  important  laws  of  the  evolution  of  organized  beings.2 
This  law  has  often  been  debated  and  applied,  but  I  do  not  know 
that  anyone  has  attempted  to  set  it  forth,  basing  his  exposition  on 
Dollo's  own  works.  This  is  what  1  propose  to  do  here,  adding  to 
my  account  some  critical  remarks  on  the  value  of  the  law  in  ques- 
tion. 

The  law  of  irreversible  evolution  was  stated  by  Dollo  as  follows: 

An  organism  can  not  return  even  in  part  to  a  previous  condition  already 
passed  through  [deja  realize]  in  the  series  of  its  ancestors.  (Dollo.  4  p.  165. )3 

It  is  usually  supposed  that  the  law  thus  expressed  applies  only 
to  parts  and  organs  which  are  reduced  or  eliminated;  but  this  is 
not  correct.  The  law  is  much  wider  in  its  application,  covering 
functional  organs  as  well.  In  order  to  understand  more  clearly  the 
far-reaching  nature  of  Dollo's  law  we  must  make  certain  distinc- 
tions in  the  concept  of  organic  evolution  and  give  some  definitions 
of  them. 

Organic  evolution  may  be,  as  we  know,  progressive,  regressive, 
and  mixed.1  If,  during  mixed  evolution  (which  is  the  most  wide- 

1  Translated,  with  permission,  by  Gerrit  S.  Miller,  ir.,  from  Science  Progress    January, 
3019. 

2  He  previously  stated  this  law  in  1892,  in  his  Cours  autographic,  etc.   (Dollo,  1),  the 
same  year  in  a  note  which  appeared  in  the  Bulletin  do  la  soc.  beige  do  Gool.,  etc.   (Dollo, 
2)   and  in  an  article  which  appeared  in  A.  Giard's  Bulletin  scientifique  do  la  France  et 
de  la  Belgique   (Dollo,  3). 

3  Later,  Dollo  expressed  his  law  with  greater  exactitude  : 

"An  organism  never  exactly  renews  a  previous  condition,  eveu  if  it  finds  itself  placed 
in  an  environment  identical  with  one  through  which  it  has  passed.  But,  by  virtue  of  the 
indestructibility  of  the  past,  it  always  retains  some  trace  of  the  intermediate  stages  which 
it  has  traversed."  Dollo  17,  p.  107,  and  10.  p.  443.)  Let  us  note,  that  Dollo  "definitely 
admits  the  reversibility  of  conditions  of  existence  :  ;'  Evolution  is  irreversible  as  regards 
the  structure  of  organisms  *  *  *,  but  reversible  as  regards  environment.  (Ethology)  " 
(Dollo,  7,  p.  15.) 

1  Tn  my  course  of  independent  lectures  (on  universal  evolution)  given  at:  the  Sorbonne 
this  year,  which  will  later  be  published,  I  have  defined  evolution  in  general  as  follows: 
"Evolution  is  a  thing's:  coming  into  being  by  successive  stages  of  change."  When  each 
successive  stage  of  the  evolutionary  process  contains  something  more  than  the  preceding 
stage,  evolution  is  progressive;  it  is  regressive  when  each  successive  stago  Contains  some- 
thing less  than  the  preceding.  Evolution  is  mixed  when  in  an  evolving  whole  one  n;u-1 
evolves  progressively  and  the  other  regressively. 

429 


•!•')()  AXXl'AL    RKroiiT    S.MITMSOXIAX     1  "XSTITI'TK  *X,    IM^. 

-pread  type  in  the  domain  of  organic  evolution)  progression  pre- 
dominates, or,  to  put  it  in  another  way,  if  the  final  condition  reached 
rcpre.-enl.-  progress  in  comparison  with  the  initial  condition,  \ve  then 
•  hal!  ca.il  such,  a  mixed  evolution  "ascending  evolution,  and  of 
I  hi:-  process  the  extreme  type  is  represented  b\  pure  progressive 
evolution  with  or  without  the  addition  of  nev>  parts.  !>ut  it,  in 
'•nixed  evolution,  regret-ion  predominates,  or,  in  oilier  words,  if  the 
I'ma!  condition  reached  i^  a  repress  in  comparison  with  the  initial 
condition,  \ve  i-hall  call  mixed  evolution  of  this  kind  "descending 
evolution."  pure  regressive  evolution  evidently  representing  ihe  ex- 
nvme  type  oi:  Hich  a  process.  Tlie  foot  ci  the  In-  ide  <>f  a 

.--in. 'lie  di:.''it.  v\'h.ich  bar  come  from  a  [)entadactyle  loot,  hy  the 
atroph\  of  tiie  lateral  di<iii>  and  the  o-i'eal  increase  of  the  median 
dipl.  i-  the  best-known  example  of  ascending' mixed  evolution,  while 
iht>  skull  of  ihe  living  Ccratodii",  in  coni[)arison  with  that  of  Dip- 
leru>.  H-  [)i:obahle  Devonian  ancestor.  [)re-ent,s  an  example  of  de- 
seen  din?/  mixed  evolution. 

Taking  into  consideration  on  (he  one  hand,  the  definitions  which 
we  have  just  made,  and,  on  the  other,  the  example-  cited  beyond,. 
which  Doilo  brought  forward  in  favor  of  his  law,  we  ou.idit  to  sep- 
arate  the  cases  of  ascending  evc^ntion  from  those  of  descending 
evolution,  .-on set  him;'  which  Dollo  himself  did  nor  do.  Clearly, 
if  the  structure  of  an  orii'jui  or  if  the  parts  of  an  organ  are  lost 
through  <//•*<  evolution,  and  if  it  is  not  possible,  as  is  almost 

iniuioush  re]  >lace  the  lo.-t  -I  rues  lire  or  part.--,  it  does  not 

ist  a  priori — iliat  a  reversal  of  evolution  would 
not  ::''  contrary  case:    that  is,  when  the  structure  of 

;ii-  oi'i»'a!  <>  lost   hy  'lie  o.HC.cndhuj  CN'oiutjou  of  tins  organ. 

refoj'e  replace  Dollo'-  -ingle  Jaw  by  three  ditferent 
.  •••   •  •  lir.-t  and  most  fundamental,  will  express  the 

y  <>!'  a  rcac(juisit  ion  of  inst  j)aii-,  the  second  of  which 
\\ill  >ppl\  to  the  ease.-  in  which  tlie  original  structure  of  an  organ 
ii-i-  -cending  evolution,  and  the  third  to  the  cases  in 

\\hi  •  has  hcen  lo-st  by  descending  e\'olution. 

;  i  -    i'o  i  1  < ;  vv  -  ; 

of  organs  reduced  or  lo.-j.  through  regressive 
ca        •  ;   '  e  regained  o\r  a  new  pro^'resHve  evolution.1 
:/.    if   the   original    structure   of   an    organ    ha-    been    in.-i    through 
•voli  or   without    ihe   addition    of   new   ]>art.s) 

i  he  '  i'co'ii  mi  <  i  : 

•  i  ie   !'  >s1    |i;i  rt :  [ii  isil  i:  >n  being 

iUlDO:-    iblc    ;,rr.,rdiu'/    !o    ihe    first     !:i\V. 


IRREVERSIBLE  EVOLUTION — PKTROS'tP^  TC'S. 

(b)  By  the  regressive  evolution  of  the  new  parts,  the  total  disap- 
pearance of  these  parts  being  impossible. 

(c)  By  the  ascending  evolution  of  these  new  parts  in  a  ne\v  direc- 
tion. 

3.  If  the  original  structure  of  an  organ  has  been  lost  through  de- 
scending evolution  (with  or  without  the  loss  of  parts),  this  original 
structure  can  not  be  regained : 

(i)  By  the  reacquisition  and  progressive  evolution  of  the  lost- 
parts,  this  reacquisition  being  impossible  according  to  the  first  la\v. 

(ii)  By  the  ascending  evolution  of  the  nonreduced  parts  in  a  new 
direction. 

(iii)   By  the  ascending  evolution  of  altogether  new  parts. 

II. 

The  various  cases  falling  under  these  three  laws  we  wish  now  to 
explain  by  examples  found  in  the  writings  of  Dollo. 

For  the  first  law  examples  are  very  numerous.  The  birds  lost  their 
teeth  during  the  Cretaceous  period;  no  subsequent  bird  has  been 
able  to  regain  these  lost  parts.  The  mandible  of  mammals  consists 
of  a  single  piece  homologous  with  the  dentary  part  of  its  reptilian 
ancestors;  no  mammal  has  been  able  to  regain  the  lost  other  part., 
of  the  reptilian  jaw,  etc. 

But  the  examples  that  especially  demonstrate  the  validity  of  the 
first  law  are  those  in  which  the  return  to  ancestral  conditions  would 
necessitate  the  reappearance  of  parts  which  an  organism  has  lost. 
As  these  examples  are  at  the  same  time  illustrations  of  the  two 
other  laws  we  shall  deal  with  them  in  connection  with  these  laws. 

The  best-known  example  of  the  first  alternative  under  the  second 
law  is  the  pseudo  dentition  of  Odontopteryx,  an  Eocene  fossil  bird. 
Instead  of  the  true  teeth  that  have  been  lost,  Odontopteryx  has  the 
margin  of  the  beak  and  of  the  lower  mandible  dentate  like  a  saw. 

The  most  striking  example  of  the  second  alternative  under  the 
second  law  is  the  pelvis  of  Triceratops.  The  dinosaurian  ancestors 
of  Triceratops  had  become  adapted  to  bipedal  life,  and  therefore 
were  possessed  of  a  very  long  and  very  narrow  ischium  and  of  a 
pubis  provided  with  a  postpubis  which  was  similarly  very  'long  and 
very  narrow  (Dollo.  10,  p.  4-1-1).  In  its  secondary  adaptation  to 
quadrupedal  life  Triceratops  was  not  able  morphologically  to  regain 
the  triardiate  pelvis  of  its  far-distant  quadrupedal  ancestors,  for  it 
has  retained  traces  of  the  bipedal  phase  in  the  rudimentary  posi- 
pubis  and  in  the  narrow,  recurved  ischium.  That  is  to  say,  the  post- 
pubis. the  new  structure  acquired  during  bipodal  life,  could  not 
totally  disappear,  and  the  new  form  of  the  ischium  could  not  dis- 
appear either  (Dollo,  10,  p.  440). 

The  most  important  and  most  obvious  example  of  the  third  alterna- 
tive under  the  second  law  is  also  found  in  a  dinosaur,  nearlv  related 


4-'}^  ANN'I-.\I,    ;;:-.r<M;T    r-MITUSOXIAV    IKSTTTTTIOX,    101S. 

to  the  preceding.  S(.>ifosaurus.  This  animal  had  for  its  immediate 
aii'.TStors  bipeds  like  "he  ance.-tors  of  Trieeralops.  and,  like  Tricera- 
lops.  it  became  readapted  to  ouadrupedal  life,  nut  \vhile  the  tri- 
radiate  pelvis  of  its  far-away  quadrupedal  ancestors  lias  been  physio- 
logically reestablidied  ihroiiirli  regressive  evolution  (atrophy)  of 
poMpubis  and  ischium  in  Triceratops  (  Dollo.  1.  c.  p.  !•-!-<>)  these 
{)art>  have  evolved  iii  ;i  IK:\V  direction  in  SLeir(»-:anrii  .  Here  the 
i- chili  in  i)i'co!iir-  *  in']  II-I^IM!  :,  ii;  S  Hat  leneil :  tlie  ]K>stpiib;  •  ihe  same 
and  moreoxer  applies  Ux-ir  eiosely  alonir  tlie  ventral  in  ii\a'in  o['  the 
ischiuin.  l>ui  moi'piioloii'ically  tliere  i-  here  no  retn.rn  to  the  former 
triradiatc  cinidition  of  the  [)elvis.  since  th«'  i>chinm  has  kept  some 
I  race  of  tlie  form  which  it  acquired  in  the  biped  pha-e.  and  liie  |)O— 
terior  I.' rand)  -  • !'  !  iu-  j  •<-!  \  i-  i:-  no  Ioniser  formed  bv  the  iseh  mm  alone 
hut  b\'  thi-  •  eoinple.x.  A\'hi!e  ex-oh'inL!1  'u:  tlie  new 

diieciion  tin  I  hus  changed  in  function  (Dolio.  !.  c.  p. 

•i  IT ). 

We  find  an  i'lnsl!  :it;on  of  lluv  firs!  alternative  of  (lie  thinl  law  in 
the  evolution  of  the  arm?:  in  the  Octopods.  'These  anima  is  in  adaj)t- 
inii1  tliem>elve.-  to  •  i  a  pair  of  arm.-  (t  lie  tentac- 

ular arm-)  ;  iiale  ancestors  (he  heteropod  de- 

capods. M.  ;  L'oine  i>opod>  autam  (exc(:[)(rni!.*  I  lie  pecu- 

liar  cas<  ctocotylisation  <d  one  of  its 

arm-)  like  theii  •'•-  tlie  iH-K^nnotenthids  (isopod  de- 

eapod:-  i  \'^i:-'  1  he  same  number  of  arms 

(see  Dollo.  i 

The  best  tion  for  the  second  alternative  of  the  third  hnv  is 

the  foot  •>!  ilagns.  an  arboreal  IvMn.ii'ai'oo.  The  si  r  net  u  re  of 

the  fool  in  the  ial  Macropodidae-  the  predominance  of  the 

fourth  to  second  and  third,  the  reduction  of 

ihe  lil'i',  li  ipearance  of  the  ii'reat  toe  slio\\"s  us 

thai  •  •  \\i.M'e  arboreal.  In  Dendrolainis,  a 

Maci'opni  arboreal,  llie  opposable  ti'roat 

toe.  co!,;:  •  diaie  ancestors  t!ic  terrestrial 

kaiiii'ai  I  !.!•.  i>iit  the  unreduced  |)arts  of 

the  I'  l»'i  n^'  evolution  ni  :<  new  direction. 

\\'iii!"  i  he  in.  i  ii  :  iminished  in  length, 

ihe-  ph;i  e  ieniilhened  and  tiie  chnvs  !ia\e 

;!1  the  same  time  !  Tim-  the  font  of  Dendroiairus  has 

tioi  bei  r  i  rucl  me  of  the  foot  ol  il>  distant 

icci  :i  11  v    flu" 
I  litmtfifirnini*  afriratm*.   in    I'I 


IRREVERSIBLE   EVOLUTION PETRONIEVICS.  433 

ancestors,  which  possessed  an  opposable  great  toe,  syndactyly  of  the 
second  and  third  toes,  dominant  fourth  toe,  and  reduced  claws.  (See 
Dollo,  6,  pp.  194  and  199.) 

Finally,  for  the  third  alternative  of  the  third  law  we  have  an 
illustration  in  the  secondary  carapace  and  plastron  of  the  turtle 
Dermochelys.  The  distant  ancestors  of  this  turtle  were,  like  it,  sea- 
turtles;  its  reduced  primary  plastron  (a  ring  formed  of  four  pieces) 
and  its  still  more  reduced  primary  carapace  (represented  by  the 
nuchal  plate  alone)  bear  witness  to  the  fact.  When  adapting  them- 
selves to  littoral  life  the  immediate  ancestors  of  Dermochelys  reac- 
quired  a  carapace  and  a  plastron,  but  this  carapace  and  plastron  are 
entirely  new  structures  of  superficial  dermal  origin.  Eeadapting 
itself  to  marine  life  Dermochelys  has  preserved  this  carapace  and 
plastron  of  its  immediate  ancestors,  although  both  are  already  much 
reduced.  (See  Dollo,  7,  pp.  9-14.) 

ITT. 

The  importance  of  the  law  of  irreversible  evolution  is  multiple. 
In  the  first  place,  this  law  has  a  phylogenetic  application,  that  is,  it 
places  us  in  position  to  reconstruct,  with  the  often  insufficient 
paleontological  material  which  we  possess,  phylogenetic  series  which, 
if  they  are  not  true  series  are  at  least  series  which  represent  indubi- 
table evolutionary  stages.  Its  ethological  application  is  yet  more 
considerable.  It  is  often  the  only  means  of  determining  the  con- 
ditions of  existence  and  the  method  of  adaptation  to  life  of  fossil 
organisms.  But  sometimes  this  law  has  a  morphological  importance 
also,  because  by  using  it  we  can  distinguish  homologies  from  mere 
analogies.  Finally  Dollo  has  shown  that  it  can  act  also  as  a  guide 
in  classification,  that  it  therefore  has  a  systematic  application. 

The  most  important  phylogenetic  application  of  the  law  was  made 
by  Dollo  in  the  difficult  question  of  the  phylogeny  of  the  Dipnoi. 
Dollo 's  very  ingenious  paper  on  this  subject  (Dollo,  5)  should  be 
considered  a  model  presentation  of  the  philosophic  point  of  view 
in  the  new  paleontology.  Before  Dollo  this  subject  was  in  a  truly 
chaotic  state,  one  of  the  most  eminent  paleontologists  having  de- 
clared Dipterus,  the  oldest  and  most  primitive  type,  to  be  the  most 
specialized.1  Nowhere  else  has  the  conception  of  the  irreversibility 
of  evolution  given  such  brilliant  results.  Since  this  conception  ex- 
presses the  idea  that  we  never  fully  return  to  an  ancestral  structure 
it  can  be  used  to  determine  whether  a  particular  condition  is  primary 
or  secondary.  Consequently  it  can  be  used  to  decide  upon  the  direc- 
tion of  evolution  when  we  have  a  series  containing  a  sufficient  num- 

i  See  A.  S.  Woodward,  Catalogue  of  the  Fossil  Fishes  iu  the  British  Museum,  pt.  2, 
1891,  p.  XX.  But  after  Dollo  published  his  important  memoir  Woodward  accepted  his 
conclusions.  See  his  presidential  address  to  the  Section  of  Geology,  in  jSature,  vol.  81, 
1909,  p.  292  (and  Dollo,  1(J,  reni.  2,  p.  387). 


434  ANNUAL  REPORT  SMITHSONIAN   INSTITUTION,   11)18. 

ber  of  terms  intermediate  between  the  extremes  (Dollo,  5,  p.  971). 
Just  such  a  series  we  possess  in  the  paleontological  series  of  the 
Dipnoi:  Dipterus  valenciennesi,  Dipterus  macropterais,  Scaumen- 
acia,  Phaneroplenron,  Uronemus,  Ctenodus,  Ceratodus,  Protopterus, 
Lepidosiren  (1.  c.,  p.  88).  Dollo  shows  that  the  structure  of  the 
tail  as  well  as  that  of  the  top  of  the  head,  the  squamation,  the  pugu- 
lar  plates,  the  opercular  apparatus,  the  ganoin,  and  the  ossification 
of  the  mandible,  the  suborbital  band — all  this  proves  that  the  course 
of  evolution  has  been  in  the  direction  from  Dipterus  to  Ceratodus. 
and  not  the  opposite  (1.  c.,  p.  89-97).  It  is  especially  by  the  struc- 
ture of  the  tail  that  the  concept  of  irreversibility  is  illustrated.  In 
a  long  and  thorough  treatment  of  the  subject  Dollo  shows  (1.  c., 
pp. .89-97)  that  the  diphy cereal  tail  of  the  Dipnoi  (and  of  the  other 
known  ancient  and  modern  fishes)  is  a  secondary  diphy  cereal  tail 
whose  morphological  value  in  the  Dipnoi  (the  second  dorsal  fin,  the 
second  anal  fin)  is  not  equivalent  to  the  morphological  value  of  the 
primitive  diphycercal  tail  (caudal  fin).  In  this  secondary  diplry- 
cercal  condition  there  is  therefore  no  return  to  the  primitive 
structure. 

The  most  important  other  cases  of  phylogeny  which  Dollo  has 
considered  are  the  phylogeny  of  the  sirenians  (Dollo  3,  p.  119),  the 
phylogeny  of  the  Leather-backed  turtle  (Dollo,  7,  p.  9),  and  the 
phylogeny  of  the  Holocephali  (Dollo,  13,  p.  107). 

One  of  the  most  important  cases  with  regard  to  the  ethological 
application  of  the  law  of  irreversibility  is  found  in  the  memoir  on 
the  Dipnoi.  If  it  be  assumed  that  Dipterus  comes  from  Ceratodus, 
as  the  latter  is  an  adaptation  to  life  in  turbid  water,  it  would  be 
necessary  to  suppose  either  that  Dipterus  represents  an  adaptation 
to  life  in  mud  (excessively  turbid  water),  or  else  that  it  represents 
a  return  to  life  in  clear  water.  The  first  alternative  being  that  of 
Lepidosiren,  the  second  is  the  only  one  which  remains  open  for  dis- 
cussion (Dollo,  5,  p.  99).  But,  putting  aside  paleontological  and 
purely  ethological  reasons,  the  law  of  irreversibility  is  sharply  op- 
posed to  such  a  view. 

"Would  the  lost  ganoin  return?  Would  the  cephalic  shield  resolve  itself 
into  its  ancestral  elements?  Would  the  suborbital  band  with  its  ossicles  in 
varying  number  become  once  more  a  solid  arch?  Would  the  opercular  appa- 
ratus resume  its  original  dimensions?  Would  the  vanished  jugular  plates  re- 
appear?" As  all  of  these  structures  are  reduced  in  Ceratodus  (1.  c.,  p.  100), 
Dipterus  can  only  represent  a  primary  adaptation  to  life  in  clear  water,  that 
is  to  say  it  is  purely  a  fish  ("  the  most  pisciform  of  Dipnoi,"  1.  c.,  p.  101). 

1  Discussing  the  subject  of  the  phylogeny  of  the  Iloloeephali  (Dollo.  13,  pp.  107-108), 
Dollo  says:  "  The  idea  of  the  irreversibility  of  evolution,  which  has  led  me  to  the  results 
that.  I  have  just  demonstrated,  has  once  more  shown  its  usefulness.  Without  it  one  would 
be  led  to  assert  that  specialized  organisms  could  become  primitive  again  and  then  once 
more  specialize  themselves  in  the  same  or  another  direction.  Such  an  assumption,  unless 
supported  by  absolutely  complete1  paleontological  series — which  we  are  far  from  possess- 
ing— would  destroy  all  possibility  of  discovering  phylogeny,  the  main  object  of  nior- 
puology." 


IRREVERSIBLE   EVOLUTION PETRONIEVICS.  435 

Another  important  instance  of  the  ethological  application  is  fur- 
nished by  the  bipedal  habits  of  the  immediate  ancestors  of  Stego- 
saurus  and  Triceratops. 

If  evolution  were  reversible  these  two  dinosaurs  would  have  exactly  regained 
their  former  quadrupedal  structure,  and  there  would  have  been  no  way  to  dis- 
tinguish their  secondary  quadrupedal  existence  from  the  first  (Dollo,  10,  p.  448). 

The  other  most  important  cases  of  ethological  application  are :  The 
secondary  adaptation  to  the  swimming  sea  life  of  the1  Pycnodonts 
(Dollo,  17,  pp.  108-9),  the  secondary  adaptation  to  the  swimming  sea 
life  of  the  Trilobites  Dephon  and  Aeglina  (Dollo  16,  pp.  410  and 
412),  etc. 

Among  the  instances  of  the  morphological  application  of  the  law, 
that  of  the  secondary  abdominal  ventral  fins  in  the  teleosts  has  a  spe- 
cial importance.  As  is  known,  the  ventrals  of  teleosts  may  be  either 
abdominal  or  thoracic  or  jugular.  But  among  the  abdominal  ventrals 
we  have  two  types — those  which  have  no  connection  whatever  with 
the  pectoral  girdle,  and  those  joined  to  the  clavicular  symphysis  by 
a  ligament.  As  there  is  no  reason  for  the  presence  of  this  ligament 
in  situ  we  have  to  conclude  that  it  is  the  degenerate  remnant  of  a 
former  direct  connection  with  the  pectoral  girdle.  In  conformity 
with  the  irreversibility  of  evolution  the  ventrals  in  again  becoming 
abdominal  have  kept  the  connection  with  the  clavicular  symphysis 
which  they  acquired  when  occupying  a  thoracic  or  jugular  position 
(Dollo,  14,  p.  139). 

The  other  important  instances  of  the  morphological  application 
of  the  law  are:  (1)  The  very  anteriorly  placed  choanae  of  the  sea 
turtles  (Dollo,  8,  pp.  817-820),  (2)  the  longirostral  and  brevirostral 
condition  in  Crocodilians  (Dollo,  12,  p.  85),  etc. 

Finally,  we  must  mention  the  one  instance  in  which  Dollo  has 
used  his  law  in  systematic  work — the  Ptyctodonts.  Before  Dollo 
these  fossil  fishes,  then  known  from  their  dental  plates  only,  had  been 
placed  among  the  Holocephali.  In  his  important  memoir  on  this 
subject  (Dollo,  13)  Dollo  showed  that,  by  virtue  of  the  law  of  ir- 
reversible evolution,  the  Ptyctodonts  can  not  be  regarded  as  Holo- 
cephali and  that  they  ought  to  be  treated  as  Arthroderes.  Since 
then  Dollo's  conclusion  has  been  wholly  confirmed. 

Although  the  empiric  evidence  for  the  validity  of  his  law  has  been 
abundant  and  varied,  Dollo  was  not  satisfied  with  such  a  wholly 
empiric  demonstration.  He  has  attempted  to  give  a  deductive  dem- 
onstration as  well.  He  says : 

The  Irreversibility  of  Evolution  is  not,  as  many  have  believed  it  to  be, 
merely  an  empiric  law  based  purely  on  facts  of  observation.  But  it  has  deep- 
seated  causes  which  carry  it  in  final  analysis  to  a  question  of  probabilities, 
us  in  the  case  of  the  other  laws  of  nature.  Evolution  being  a  summation  of 
exactly  determined  individual  variations  in  an  exactly  determined  order,  to 
have  it  reversible  would  be  to  admit  the  possibilty  of  the  intervention  of 
causes  exactly  the  inverse  of  those  which  produced  and  fixed  the  individual 
variations  from  which  the  first  transformation  arose,  and  in  an  exactly  inverse 


430  AXXUAL    RKPORT    SM  1TI 1SOX  !  AX    1  XSTITI.'TTOX,    1918. 

order — circumstances  too  complex  for  it  to  bo  imagined  that  they  are  ever 
realized.  (Polio,  19,  p.  -~9,  rein.;  sei>  also  Polio,  3,  p.  127.) 

And,  when  speaking  of  tlie  impossibility  of  the  descent  of  Dip- 
terns  from  Ceratodns  (Dollo,  5,  p.  100,  the  passage  referred  to 
above)  he  says : 

And  let  it  bo  noted  that  it  is  here  a  question  not  of  one  isolated  character, 
bin  of  a  whole  group  of  characters,  something  that  is  much  more  serious  so 
far  as  irreversibility  is  concerned  *  *  *  But  it  is  particularly  in  its 
action  on  elements  as  multiple  as  these  that  we  can  aflirm  will)  certainty  that 
evolution  is  not  reversible  (1.  c.  rem.  72,  p.  122). 

The  irreversibility  of  evolution  becomes,  therefore,  according  to 
Dollo,  the  more  probable  as  the  number  of  elements  increases,  and 
it  is  practically  a  necessity  when  the  number  of  elements  is  con- 
siderable. 

TV. 

Having  explained  the  law  of  irreversible  evolution,  the  various 
cases  which  it  makes  clear,  also  its  applications,  and  its  logical 
basis,  we  now  wish  to  make  some  critical  remarks  on  the  various 
aspects  of  the  law. 

In  the  Hist  place,  its  logical  basis.  The  deductive  demonstration 
of  his  law,  attempted  by  Dollo.  is  very  doubtful.  As  to  the  number 
of  elements  on  which  evolution  acts,  it  is  not  a  question  of  cells, 
but  of  organs  and  parts  of  organs  (because  it  is  only  these  last 
which  have  their  peculiarities  determined  in  the  germ),  and  the 
number  of  these  organs  and  parts  is  not  relatively  great  even  in  the 
most  complex  organism.  But,  if  we  consider  the  much  greater 
number  of  individuals  in  which  the  organs  and  parts  of  organs  show 
individual  variations,  the  chance  that  they  will  vary  in  different 
directions  and  consequently  also  in  inverse  directions  becomes  pos- 
-ible.  ft  is  only  if  we  assert  that  individual  variations  are  rela- 
tively not  very  numerous — predetermined — that  this  course  of  rea- 
soning founded  on  pure  probability  becomes  weak.  In  that  case, 
however,  the  law  of  irreversible  evolution  is  not  the  result  of  nu- 
merical probability,  but  the  result  of  unknown  internal  causes  of 
organic  evolution. 

There  is,  therefore,  no  logical  necessity  in  the  law  of  irreversible 
evolution,  and  this  law  remains  a  purely  empirical  rule.  Let  us 
now  -oe  how  much  the  three,  laws  of  this  evolution  are  confirmed  by 
experience,  and  to  what  extent  we  should  expect  possible  exceptions. 

As  to  the  first  law,1  this  law  appears  to  be  without  exceptions  so 
far  as  it  applies  to  lost  organs  and  parts.  For  the  loss  of  an  organ 
or  of  a  part  having  become  final  by  the  loss  of  the  corresponding 
tendency  in  the  germ,  it  is  almost  impossible  to  imagine  the  reap- 
pearance of  (hi-  tendency,  bearing  in  mind,  on  the  one  hand,  the 

tin-   similar  oJ^.-rvalions  of   A,    Hamilirsrh,    1M,    ]..    i:^s    (cilnl   by    l»ollo,    lu 
4  'JIM, 


IRREVERSIBLE   EVOLUTION PETRONIEVICS.  437 

difficulty  of  producing  new  tendencies  in  the  germ  by  the  influence 
of  external  onuses,  and  on  the  other  the  degree  of  correlation  that 
would  be  needed  among  these  tendencies.  When  it  is  a  question  of 
the  reduction  of  an  organ  or  part,  two  alternatives  must  be  distin- 
guished. If  the  reduction  has  gone  so  far  that  the  corresponding 
tendency  in  the  germ  is  verging  011  complete  disappearance,  the  re- 
duced organ  or  part  will  find  itself  practically  in  the  same  condition 
as  if  it  were  already  lost.  But  if  their  reduction  has  not  reached 
to  such  an  extreme  their  evolution  in  an  inverse  direction  will  not 
be  impossible. 

For  the  second  law  we  must  distinguish  between  the  case  of  a  single 
part  and  that  of  a  complex  organ.  The  regressive  evolution  of  a  sin- 
gle part,  if  during  this  evolution  and  the  preceding  progressive  evo- 
lution no  change  of  form  has  taken  place,  could  clearly  lead  back  to 
the  point  where  the  progressive  evolution  started.  And  the  regres- 
sive evolution  of  a  single  part,  if  the  corresponding  arrangement  in 
the  germ  is  not  too  much  enfeebled,  could  evidently  also  be  followed 
by  a  new  progressive  evolution.  But  if  a  change  of  form  has  taken 
place  during  the  first  progressive  evolution,  and  if  this  change  of 
form  has  been  so  great  that  a  change  in  the  corresponding  arrange- 
ment of  the  germ  has  been  necessary,  then  neither  the  regressive  evo- 
lution following  the  original  progressive  evolution  will  be  able  to 
lead  back  to  the  point  of  departure  of  the  latter,  nor  will  a  new  pro- 
gressive evolution  have  the  power  to  accomplish  it,  because  to  do  so 
would  necessitate  the  return  to  a  disappeared  condition.  If.  for 
instance,  a  tooth  has  first  increased  in  size  and  afterwards  dimin- 
ished without  change  of  form  this  tooth  will  be  able  by  diminution 
to  assume  the  dimensions  which  it  had  at  the  beginning  of  its  in- 
crease, and  a  new  increase  of  the  same  kind  will  not  be  impossible 
(if  the  reduction  has  not  gone  too  far).  But  if  the  increase  in  size 
has  been  accompanied  by  a  radical  change  of  form,  if,  for  instance, 
a  conical  tooth  has  become  laterally  compressed,  then  a  return  to  the 
conical  form  will  not  be  possible  either  during  its  diminution  l  or 
during  a  new  increase  in  size. 

In  the  case  of  a  complex  organ  Dollo  asserts  that  its  return  to  the 
previous  condition  through  the  action  of  regressive  evolution  is  im- 
possible on  account  of  the  "  indestructibility  of  the  past."  But  if 
a  single  reduced  part  of  an  organ  is  regarded  as  the  supposed  reason 
why  reversibility  is  impossible,  we  are  able  to  affirm  almost  with  cer- 
tainty that  in  such  a  case  the  indestructibility  of  the  past  does  not 
exist,  because  it  would  find  itself  in  contradiction  with  the  well  es- 

1  This  impossibility  is  exactly  what  W.  D.  Matthew  supposes  to  have  happened  during 
the  evolution  of  the  FcUtlac  when  he  supposes  that  the  felines  come  from  Dinictis,  a 
primitive  saber-toothed  cat  (see  W.  P.  Matthew,  "  The  Phylogeny  of  the  Feiidae,"  Bull. 
Amcr.  Mus.  Nat.  ITist.,  vol.  2S.  1910,  p.  200  s).  Scott  has  clearly  had  a  glimpse  of  the 
fact  that  this  phylogeny  contradicts  the  law  of  irreversible  evolution  <see  W.  F>.  Scott, 
28,  p.  540  s). 

1 3GG5U  °  —20— —29 


438  ANNUAL  REPORT  SMITHSONIAN  INSTITUTION,  1918. 

tablished  law  of  the  necessary  regressive  evolution  of  nonfunctional 
parts  and  organs.  The  reversibility  of  the  ascending  evolution  of  a 
complex  organ,  when  it  depends  on  a  reduced  part,  is  therefore  not 
impossible  (we  can  suppose,  for  instance,  that  the  secondary  ventrals 
of  teleosts  will  return  in  the  future  to  their  original  condition 
through  the  complete  disappearance  of  the  clavicular  ligaments). 

In  the  case  of  a  nonrecluced  part,  whose  form  has,  however, 
changed  during  ascending  evolution,  the  indestructibility  of  the  past 
.again  does  not  exist  in  the  strict  sense,  the  nonreduced  part 
being  able  to  change  its  form  again  by  a  new  progressive  evolution, 
although  the  original  condition  of  this  part,  and  consequently  the 
original  condition  of  the  organ  in  question  can  not  be  reestab- 
lished. The  pelvis  of  Triceratops  may  be  taken  as  an  example.  The 
postpubis  of  this  pelvis  exists  in  a  very  rudimentary  condition,  and 
as  rudimentary  parts  tend  to  disappear,  the  postpubis  certainly 
would  have  disappeared  if  Triceratops  had  lived  long  enough.  It 
is  therefore  only  its  recurved  ischium,  very  different  in  form  from 
the  ischium  of  its  distant  tetrapod  ancestors,  which  was  able  to  pre- 
vent Triceratops  from  recovering  its  original  pelvis. 

Finally,  if  there  is  an  ascending  evolution  of  nonreduced  parts 
(pelvis  of  Stegosaurus)  it  is  the  change  of  function  which  saves  these 
parts  from  a  regressive  evolution ;  the  indestructibility  of  the  past 
does  not  exist  here  either.  And  it  is  clear  that  the  same  reasoning  is 
also  applicable  under  the  third  law  to  the  evolution  of  a  complex 
organ. 

To  sum  up :  The  irreversibility  of  the  evolution  of  a  complex  or- 
gan depends  entirely  on  the  irreversibility  of  the  evolution  of  the 
reduced  or  nonreduced  individual  parts  which  enter  into  its  com- 
position, and  the  second  and  third  laws  are  not  without  exceptions 
in  this  respect  any  more  than  the  first;  as  we  have  seen,  it  is  the 
germinal  base  of  the  first  lawr  wrhich  underlies  the  entire  subject. 

As  I  said  at  the  beginning,  most  naturalists  know  D olio's  first  law 
only.  This  is  only  a  part  of  his  general  law,  although  the  most  im- 
portant and  most  certain  part.1  This  general  law,  in  spite  of  the  pos- 
sible exceptions,  has  an  extraordinary  importance  for  biological 
philosophy  and  evolutionary  philosophy  in  general.  Dollo  will  al- 

1  Besides  the  law  of  irreversible  evolution  Dollo  has  formulated  (see  Dollo,  4,  p.  165) 
two  other  laws — that  of  discontinuous  evolution  (before  II.  de  Vries)  and  that  of  limited 
evolution.  In  his  subsequent  writings  Dollo  has  only  rarely  touched  on  these  two  other 
laws  (on  the  law  of  limited  evolution  see  Dollo,  7,  p.  9,  Dollo.  S,  p.  813  and  p.  820, 
Dollo,  9,  p.  131  ;  on  the  law  of  discontinuous  evolution  see  Dollo,  .">.  rem.  (66),  p.  120; 
Dollo.  7,  rem.  (11),  p.  9;  and  Dollo,  17,  pp.  139-140). 


IRREVERSIBLE    EVOLUTION  -  PETRONIEVICS.  439 

ways  be  regarded,  like  Cuvier  before  him,  as  the  founder  of  a  great 
law  of  the  organic  world.1 

LITEKATUKE. 

1.  L.   Dollo,   Cours  autographic  sur   1'evolution   du   Squelette  des   Vertebras. 

Lemons  faites  a  1'Institut  Solvay   (University  de  Kruxelles)   en  1891-2. 

2.  -        -  gul-  rorigine  de  la  nugeoire  caudale  des  Ichthyosaures.     Bulletin 

de  la  societe  beige  de  Geologic,  de  Palcontologie  cl  de  llydrologie,  vol.  6, 

1892,  Proces-verbaux,  pp.  167-74. 

3.  -  Sur  la  morphologic  des  cotes.     Bulletin  scientifiquc  dc  l<i  France  et 

de  la  Belgique,  public  par  A.  Giard,  Paris,  vol.  24,  1892,  pp.  113-29. 

4.  -  —  Les  lois  de  revolution.    Bulletin  dc  la  soc.  beige  de  GeoL,  etc.,  vol.  7, 

1893,  Proces-verbaux,  pp.  164-6. 

5.  -        —  Sur  la  phylog&iie  des  Dipneustes.    Bulletin  de  la  soc.  beige  de  GeoL, 

etc.,  vol.  9,  1895,  Meinoires,  pp.  79-128. 
(3.  _         _  Les  ancetres  des  Marsupiar.x,  etaient-ils   urboricolesV  MisecUanecs 


biologiqucs  declines  au  Prof.  A.  Giard,  Paris,  1899,  pp.  188-203. 

7.  --  Sur  rorigine  de  la  Tortue  Luth  (Dermoehelys  coriacea).     Bulletin 

de  la  societe  royale  des  sciences  medic-files  et   naturclles  de  Brunettes, 
1901,  separate,  pp.  1-26. 

8.  --  Sur  revolution  des  Cheloniens  marins   (Considerations  bionomiques 

et  phylogeniques).    Bulletin  de  V  Academic  royale  de  Belgique,  Classe  des 
Sciences,  1903,  pp.  801-50. 

9.  -         -  Un   nouvel  opercule  tynipaiiique  de   Plioplatecarpus    (Mosasaurien 

plongeur).    Bulletin  de  la  soc.  beige  de  GeoL,  etc.,  vol.  19,  1905,  Memoires, 
pp.  125-31. 

10.  --  Les  Dinosauriens  adaptes  a  la  vie  quadrupede  secondaire.    Bulletin 

de  la  soc.  beige  de  GeoL,  etc.,  vol.  19,  1905,  Memoires,  pp.  441-8. 

11.  -    -  Le  pied  de  1'Amphiproviverra.     Bulletin  de  la  soc,  beige  de  GeoL, 

etc.,  vol.  20,  1906,  Proces-verbaux,  pp.  166-9. 

12.  -        —  Nouvelle  note  sur  les  reptiles  de  1'Eoeene  inferieur  de  la  Relgique  et 

des  regions  voisines  (Eosuchus  Lerichei  et  Eospliargis  gigas).     Bulletin 
de  la  soc.  beige  de  Geol.,  etc.,  vol.  21,  1907.  Proces-verbaux,  pp.  81-5. 

13.  —    —  Les  Ptyctodontes  sont  des  Arthoderes.     Bulletin  de  la  soc.  beige  de 

GeoL,  etc.,  vol.  21,  1907,  Memoires,  pp.  97-108. 

14.  -          -   Les    Teleosteens    a    ventrales    abdominales    secondaircs.      \'crlinnd- 

lungen  der  k.  Ic.  zool.-botan.  Gescllscliait  in  Wien,  vol.  59,  1909.  pp.  185-40. 

15.  --  Les  poissons  Voiliers.     Zoologische  Jahrbiielier,  Abtlieilung  fiir  Sys- 

tcmatik,  vol.  27,  1909,  pp.  419-38. 

16.  —     —  La  Paleontologie  ethologique.    Bulletin  de  la  soc.  beige  de  GeoL,  etc., 

vol.  23,  1909,  Memoires,  pp.  377-421. 

17.  -        -  Les  Cephalopodes  adaptes  a  la  vie  neetiquo  secondaire  et  a  la  vie 

benthique  tertiaire.     Zoologisclie  Jaltrbiicbcr,  Supplement  vol.  15    (Fest- 
schrift, J.  W.  Spengel,  vol.  1),  1912,  pp.  105-40. 

18.  -       —  Globidens  Fraasi.    Archives  de  Biologic,  vol.  28,  191.3.  pp.  009-20. 

19.  -         -   Podocnemis    congolensis    et   Involution    des    ClielonitMis    fluviatiles. 

Annalex  du  Muxee  du  Congo  Beige,  Bruxelles,  1913,  pp.  49-65. 

20.  A.  Giard,  Les  tendances  actuelles  de  la   morphologic.     Kevin  •  scientilique, 

1905,  pp.  171-2. 


1  To  the  ]i^t  of  Dollo's  writings  on  the  law  of  irreversible  ovolution  I  :im  adding  the 
titles  of  some  other  papers  (most  of  them  already  cited  by  Dollo)  whoso  authors  have  dis- 
cussed or  applied  this  law.  Among  these  treatises  the  important  book  by  O.  Abel, 
Grundzugo  der  Palaeobiologie  der  Wirbeltiere,  1912,  deserves  special  mention  because  it 
contains  nearly  all  the  examples  cited  by  Dollo  and  many  others  besides,  Dollo's  law  is 
discussed,  pp.  fil«-01K. 


440  ANNUAL   KKPOPiT   SMITHSONIAN    INSTITUTION,   1018. 

21.  G.  v.  Arthaber,  Boitrage  zur  Kenntniss  der  Organisation  und  der  Anpas- 

sungserselieinungen   des  Genus   Metriorliynchus.     Beitrage  xur  Palwon- 
toloyie  mid  Geoloyie  Oest&rreicJi-Unffarns  unfl  des  Orients,  1906,  p.  301. 

22.  CH.  Depeivt,  I.os  transt'ormations  clu  inoncle  animal,  1907,  pp.  243-4. 

23.  F.  v.  Hueiu1,  Die  Dinosaurier  der  europaisclien  Triasformation,  1908. 

24.  A.    Handlirsch,    Die   fossilen    Insekten    und    die   Phylogenie   der    rezenten 

Forinen,  1906-8,  vol.  2,  p.  1328. 

25.  H.  F.  Osborn,  The  Age  of  Mammals,  1910,  p.  34. 

26.  K.    Diener,    Paheontolo.uie    und    Abstammungslehre,    Sammlung    Goschen, 

pp.  98,  110-14. 

27.  O.  Abel,  Grundziige  der  Palaeobiologie  der  Wirbeltiere,  1912. 

28.  W.  B.  Scott,  A  History  of  Land  Mammals  in  tin1  Western  Hemisphere,  ]913. 

29.  R.  S.  Lull,  Organic  Evolution,  A  Textbook,  1917,  p,  280  and  p,  572. 


of  irrevers*- 
it  I  on 


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